OK, let’s see how tiny the pixels on a screen need to be to make it a retina display for you. To do this, we’ll calculate the smallest pixels that you can resolve at a given distance. For example, if you have 20/20, or 1 arc minute, vision and hold a smartphone 11 inches (28 cm) away, you’ll be able to resolve individual pixels if there are 313 pixels per inch (123 pixels/cm) or fewer; if it has more pixels than that per inch/cm (i.e. higher pixel density & smaller pixels), then it’s a “retina display”.

Here’s how to calculate the minimum number of pixels per distance to match your eyes (fill in your visual resolution in place of “1“:

In more detail: to calculate the pixel size, s, opposite the viewer divide the angle, a, in half to give a right triangle with the viewing distance, d, adjacent to the angle and the length of ½ of a pixel opposite. 1 arc minute = 1/60 degree. Then with basic trigonometry:

A 42-inch diagonal, full HD television (1920×1080) also happens to have 52 pixels per inch; therefore, when viewed from 5-½ feet or farther the pixels begin to blur together for 20/20 vision. Homework: how close/far should you sit from your television to turn it into a “retina” display? Enjoy!